Enveloped animal viruses attach to and enter the host cell via the interaction of viral proteins in the virion membrane (envelope proteins) and cell surface proteins (virus receptors). Receptor recognition and binding are mediated by the surface envelope protein. Virus entry is an attractive target for anti-viral treatment; numerous drugs that are designed to block virus attachment or membrane fusion have been or are currently being evaluated in preclinical or clinical studies (Richman, 1998; PhRMA, 1999; Stephenson, 1999). For example, the attachment inhibitor SCH-D (vivriviroc), which blocks the interaction between viral membrane proteins of HIV-1 and the cellular co-receptor CCR5, is currently being evaluated in clinical studies for its effectiveness as an anti-viral treatment (Shurman, 2004). Other entry inhibitors currently under investigation include UK-427857 (maraviroc; Pfizer), TNX-355 (Tanox Inc.), AMD-070 (AnorMED), Pro 140 (Progenies), FP-21399 (EMD Lexigen), and BMS-488043 (Bristol-Myers Squibb). One entry inhibitor, T-20 (enfuvirtide; Roche/Trimeris), has been approved for treatment of HIV infection by the United States Food and Drug Administration.
As these drugs continue to be developed and enter the clinic, assays are needed that can rapidly and easily detect the emergence of viruses with reduced susceptibility to entry inhibitors. Also needed are methods to determine whether a virus can enter cells expressing particular co-receptors to determine whether inhibition of virus-co-receptor interactions might be useful in the treatment of infection. These and other unmet needs are provided by the present invention.